The deprecation API is to be used to retrieve information about different
cluster, node, and index level settings that use deprecated features that will
be removed or changed in the next major version.

As is shown, there is a level property that describes the significance of the
issue.

warning

You can upgrade directly, but you are using deprecated functionality
which will not be available or behave differently in the next major version.

critical

You cannot upgrade without fixing this problem.

The message property and the optional details property provide descriptive
information about the deprecation warning. The url property provides a link to
the Breaking Changes Documentation, where you can find more information about
this change.

Any cluster-level deprecation warnings can be found under the cluster_settings
key. Similarly, any node-level warnings are found under node_settings. Since
only a select subset of your nodes might incorporate these settings, it is
important to read the details section for more information about which nodes
are affected. Index warnings are sectioned off per index and can be filtered
using an index-pattern in the query. Machine Learning related deprecation
warnings can be found under the ml_settings key.

The following example request shows only index-level deprecations of all
logstash-* indices: